Biometric images used as authentication means include fingerprints, irises, and the like. Take fingerprints as an example. The image data (check image) of an unknown fingerprint is compared/collated with the image data (registered image) of a fingerprint which is stored in a database or the like in advance and associated with a specific individual. If it is determined that they are identical to each other, this unknown fingerprint image belongs to the specific individual.
In this fingerprint authentication, the magnitude of quality of a fingerprint image greatly influences authentication accuracy. In addition, the image quality of fingerprint images changes depending on differences in skin condition among individuals and the ambient environment such as temperature and humidity under which fingerprints are sampled. For this reason, to select images to be registered, from images sampled in advance, so as to store them in a database or the like, as well as to perform fingerprint authentication, the technique of evaluating the quality of obtained fingerprint images and selecting and registering images with high quality in advance is an effective means from the point of view of an improvement in authentication accuracy.
A conventional fingerprint collation apparatus will be described with reference to FIG. 11.
A conventional fingerprint collation apparatus 101 is comprised of a sensor unit 102 which captures a fingerprint image, an image processing unit 103 which performs image processing for the fingerprint image input from the sensor unit 102, an image quality evaluation unit 104 which evaluates the image quality of the image data input from the image processing unit 103, an image quality determination unit 105 which determines, on the basis of the evaluation result on the image quality which is input from the image quality evaluation unit 104, whether the image quality is good, and determines whether or not to store the image data input from the image processing unit 103, a registration image storage unit 106 which stores the image data input from the image quality determination unit 105, a check image storage unit 108 which stores check fingerprint image data for authentication, and an image collation unit 107 which compares/collates the registration fingerprint image data stored in the registration image storage unit 106 with the check fingerprint image data stored in the check image storage unit 108, and output the result.
In the fingerprint collation apparatus 101, a technique of determining image quality by using the number of characteristic points (minutiae) in fingerprint patterns such as end points or branch points has been reported as a technique of evaluation and determination on image quality which are performed by the image quality evaluation unit 104 and image quality determination unit 105 (for example, Japanese Patent Laid-Open Nos. 8-129644 and 8-263658).
In addition, a technique has been reported, which performs determination on image quality by using contrast information such as the proportion of black pixels in a fingerprint image or the level differences between the bright and dark portions of the fingerprint image (for example, Japanese Patent Laid-Open Nos. 8-110949 and 2000-222556).